U.S. Pat. No. 3,496,401 described the basic mechanism underlying the operation of tungsten-halogen incandescent lamps with specific reference to tungsten-iodide lamps. The thrust of that patent was to provide alkaline earth metal aluminosilicate glass compositions which would be suitable as envelopes for such lamps. Those glasses consisted essentially, expressed in weight percent on the oxide basis, of 10-25% alkaline earth metal oxide, 13-25% Al.sub.2 O.sub.3, 55-70% SiO.sub.2, 0-10% B.sub.2 O.sub.3, and less than 0.1% alkali metal oxide.
U.S. Pat. No. 3,978,362 disclosed an improvement upon the glass compositions recited in the above patent such as to render them especially useful as envelopes for tungsten-bromine incandescent lamps. Thus, the glasses exhibited a strain point above 700.degree. C., a coefficient of thermal expansion (0.degree.-300.degree. C.) of about 48-55.times.10.sup.-7 /.degree.C., a mismatch with molybdenum metal at the set point of the glass not exceeding about 250 PPM (parts per million), an operable melting temperature no higher than 1550.degree. C., a viscosity at the liquidus of at least 100,000 poises, and a liquidus temperature less than 1200.degree. C. Glasses demonstrating those properties consisted essentially, expressed in weight percent on the oxide basis, of about 14-21% CaO, 0-5% MgO, 0-7% BaO, the total CaO+MgO+BaO being at least 19%, 13-16% Al.sub.2 O.sub.3, 0-10% SrO and/or La.sub.2 O.sub.3, and 58-63% SiO.sub.2.
U.S. Pat. No. 4,060,423 described another group of glass compositions particularly designed for use as envelopes for tungsten-halogen lamps. Those glasses are characterized as exhibiting a liquidus temperature no greater than 1250.degree. C., a strain point of at least 725.degree. C., and a coefficient of thermal expansion (0.degree.-300.degree. C.) of about 42-48.times.10.sup.-7 /.degree.C. The compositions therefor consist essentially, expressed in weight percent on the oxide basis, of 55-68% SiO.sub.2, 15-18% Al.sub.2 O.sub.3, 6-13% CaO, and 6-16% BaO, wherein the weight ratio Al.sub.2 O.sub.3 :CaO+BaO is about 0.6:1 to 1:1.